1. Field of the Invention
The present invention relates to coupling apparatus for coupling a downhole tubular (such as a borehole casing) to a pump, conventionally known as a circulating head. The invention also relates to a method of pumping fluid into a downhole tubular, and to a method of drilling.
2. Description of the Related Art
A conventional circulating head is described by LeFleur et al in U.S. Pat. Nos. 5,282,653, 5,152,554 and 5,348,351. As described by LeFleur et al, when casing is being run in connection with the drilling of an oil or gas well, it sometimes becomes necessary to connect surface pumping equipment to circulate drilling fluid down the well. Typically, this need arises when a tight spot is encountered and drilling fluid is circulated down the well to run the casing past the tight spot and avoid the need for removing the casing and redrilling the hole.
The circulating head described by LeFleur et al uses a segmented ring of eight dogs to lock a cylindrical member in place. The dogs are fixed in place by rotating a bottom end cap.
This arrangement suffers from a number of problems. Firstly the apparatus has a large number of working parts. Secondly it can be difficult and time consuming to rotate the bottom end cap.